It is well-known that cutaneous areas called dermatoms are distributed over the human body, which are connected to individual spinal segments. Excitation by electrical means of such cutaneous areas is known from U.S. Pat. No. 4,570,640 to Barsa, hereby included by reference into the present disclosure, and where the localizations of human dermatoms are explained and illustrated, including their respective conventional denominations. The purpose of electrical excitation of dermatoms in that disclosure is to determine the level and depth of nerve blocks, particularly for controlling anesthesia, the patient signalling by himself or through physiological measurement means the effect of excitations of varying strength. There is, however, no possibility disclosed for measuring the time of transport of nerve signals.
In the present case, however, the object is to determine the functionality of aspects of the nervous system, and in particular to diagnose conditions of spinal malfunction as evidenced from dermatom somatosensory evoked potentials (DSEP) and their time dependence.
In the prior art DSEP testing, an electrode pair is applied to a cutaneous area representative of a dermatom and electrical pulse signals are applied. The responses as received as cortex signals from the head are registered as related to the times of pulse application, summing a number of such signal waveforms in order to eliminate noise, the waveforms from individual pulses being heavily disturbed by noise. After registering about 150 waveforms, another dermatom is chosen, and a new set of waveforms is registered and summed. After registering results from a number of dermatoms, it is then necessary to repeat the operation, since it is established and wise practice not to trust an observation which does not have a guarantee of reproducibility. It is hardly possible to use a pulse repetition frequency in excess of about 3 per second, and each registering for a dermatom would therefore demand about one minute. A normal examination would need to operate on about 8 dermatoms (the human body having over 50 dermatoms), and the doubling of the procedure as presently needed would therefore, in the best of cases, take about 16 minutes. However, it is unlikely that a patient would be in the same state in relevant respects during such a time, the state of mind, variations in boredness, drowsiness, heaviness being likely to change the reactions. It should be kept in mind that the patient is in general not normally fit but likely to tire easily.